In optical fibers and optical components the refractive index of the materials used to manufacture the fibers and components is important as it governs the speed at which light travels through the medium. In certain applications it is necessary that the refractive index of the medium be altered selectively. In many of the optical fibers used in optical communications systems the refractive index can be permanently modified through exposure to ultraviolet light. This technique is used to adjust the effective optical path length in devices such as fiber Mach-Zehnder or Michelson interferometers or to write gratings such as fiber Bragg gratings. U.S. Pat. No. 5,367,588 which issued Nov. 22, 1994 to Hill et al. describes the inscription of fiber gratings and U.S. Pat. No. 4,900,119 which issued Feb. 13, 1990 to Hill et al. discloses an all fiber Mach-Zehnder application.
Fiber Bragg gratings are used in optical systems in such applications as; amplifiers, filters and add/drop (de)multiplexers. Fiber gratings are typically written by selectively exposing an optical fiber to a periodic pattern of intense ultraviolet light. The exposure creates a permanent increase in the refractive index of the fiber to produce a fixed index modulation in accordance with the exposure pattern. This fixed index modulation is known as a grating. In a fiber a small amount of light is reflected at each periodic refraction change. All of the reflected light signals combine coherently to one large reflection at a particular wavelength when the grating period is equal to one half the wavelength of a light signal carried by the fiber divided by the effective index of refraction. This wavelength is called the Bragg wavelength.
In the process of fabricating optical components, such as Bragg gratings, by ultraviolet irradiation it is common to expose the fiber with a high intensity ultraviolet laser. The intensity of the laser beam striking the fiber needs to be large in order to achieve the necessary photo-induced refractive index change. Typically, a cylindrical lens that focuses the light along a line parallel to the fiber axis is used for this purpose. While the fiber must be close to the focal point of the lens to benefit from a large enhancement of intensity, it must not be too close to the focal point or else a reduction of the fiber mechanical strength will occur through ultraviolet induced defect creation in the fiber material. In an extreme case this can lead to catastrophic damage, such as breakage, to the fiber itself. If no special means are available to help, it is quite difficult to control the light intensity at a desired, reproducible level.
Typically, attempts to solve the above problem have relied on standard text-book optical techniques in which the focal point of the lens is determined using a burn pattern and then a micrometer driven stage is used to position the fiber at a known distance from the focal point. In this technique, the laser beam intensity at the fiber is known only approximately. As well the whole fiber surface is subjected to the irradiation.